Open-listening, or speakerphone, devices are known for telephone circuits (telephone speaking circuits, intercom systems, radio devices etc.) comprising an amplifier circuit as an open-listening (speakerphone) output stage and a loudspeaker. Unipolar amplifier circuits with asymmetrical output (having an amplifier element and a coupling capacitor for isolating the direct current) have not only less complexity and a smaller space requirement than symmetrical open-listening output stages but also the advantage of greater current efficiency, which means that with the commonly used loudspeaker impedances of 30 to 100 .OMEGA., they have a greater output power in the critical range of low supply currents.
When using these open-listening output stages, it is however a disadvantage that on switching on the loudspeaker a loud and disturbing clicking noise can be heard that is caused by the high current surge when the coupling capacitor is suddenly charged.